Photographing the night sky is extremely rewarding because you often get
views of things that your own eyes either can't see, or you don't think to
look for them. When seeing photos of stars, star trails, or other night
images, people are often surprised by the unrealor surrealcolors.
They often think these photos are either fake or manipulated. While that
could be the case, what most people don't understand about light is that
the human eye doesn't discern between the chemical compounds that make up
light. Humans view a very narrow spectrum. Cameras, on the other hand,
can pick up light wavelengths that is composed of chemical compounds like
helium, hydrogen, and so on. These chemicals change the "color" of light
that that we can't easily see, but cameras can.

Because our everyday world doesn't have a lot of things that are heavily
weighted towards these things, most everyday pictures that we take appear
much like we see them in real life. But it so happens that stars in
the sky are differentthey are made up of gasses that are comprised of
these chemical compounds that emit colors that we can't see, but cameras
can. So, the "hues" you may see in photos of stars may appear vivid,
but that does not mean it's been artificially altered.

When photographing stars, you can either get a star "field," a static
snapshot of the stars as points of light, or star "trails," where the
stars' movements streak across the sky. How long you expose the image
determines which you get. The first rule of thumb to remember is that
the Earth rotates such that the light from a star begins to "move"
after about 15 seconds. It's apparent movement is largely dependent
on your lensthe longer the focal length, the more apparent the movement;
the wider angle lenses won't show much movement till later because of the
star point is so small. For comparison, a close-up photo of the moon can
only be about 6 seconds before the Earth's movement blurs it.
Part of your experimentation will be to gauge the timing for how much
"trail" you want.

The above photos were taken at
King's Canyon National Park in California, with a
setting of 30 seconds at f2.8, using an ISO of 3200. The camera I used
is a Canon EOS 1DsMark3, however any camera that can do 30 seconds at
f2.8 should get pretty good results, provided the sky is cooperative.
A fun thing to do with these "still" photos of star feilds is to use
a cable release with an interval timer to take a series of consecutive
pictures and string them all together in a movie. In this case, I
created a video consisting of about 210 photos, each taken exactly the
same way. Since each exposure is 30 seconds long, the entire sequence took
120 minutes, or about two hours. Click on the photo to see the movie.
Once the individual frames are captured, you can use a variety of
editing tools to string them together into a video format, such as
Apple's iMovie program, which is included with the computer, or any number
of applications. Search for "movie editing software" on the net. All of
them can produce movies from still frames like this. I happened to use
a program called "FotoMagico" on my Mac.

A time-lapse movie can capture the sky moving as a series of photos, but
capturing this movement in a single picture is called a "star trails" photo.
There are two ways to do itone is by taking a single "long exposure"
photo, and the other is to take a series of photos (like above) and then
sandwich them together into one photo. Here, the consecutive sequence of
stars will mesh together to look like trails. The first method is far
easier, but not everyone's camera can do long exposures well. There are
two problems: the longer the exposure, the more noise there is in the
resulting photo. Higher end cameras don't suffer from this as much, but
those with lower-end cameras may be quite unsatisfied with the results.
The other problem is that some cameras can't do exposures longer than
30 seconds, making the multi-exposure method the only option. Of cours,e
the other side of the coin is that multiple-exposures requires dealing
with a lot of files and getting familiar with photo-editing software
in order to make the final image. So, there are upsides and downsides
to each method. This article only addresses the long-exposure
technique, not the multi-exposure sandwiching technique.

The process for taking long exposures in a single frame is technically simple.
You only need a camera that can has the "bulb" exposure setting, and a
cable release. Generally, you set the camera on the widest aperture setting
your lens can do, press the cable release, and go get coffee, or have
dinner, or go to sleep. Many star trails pictures are hours long. Once you
try this, you'll be instantly excited by the results. However, if you're
like most people, you will also find you've made a few mistakes, or that
the exposure didn't quite come out right, or that the focus is just a little
soft, or that your foreground objects (not stars) aren't what you hoped they'd
be. This is why this article goes on.

To avoid many of the common mistakes with star trails photos, the main
things to keep in mind are:

As noted above, the first problem most people find is that their exposures
don't come out quite right. They're either too dark or too light. If it's
too dark, you need to open up your aperture more to let in more light.
If it's too light, it's probably because there's ambient light in the sky
you didn't anticipate. This light may come from nearby cities, the
diminishing sunset an hour or more afterwards, or the moon (even if it
hasn't yet risen in the sky). What your eye sees is nothing compared
to a long exposure of a camera, where this residual light can be so
overwhelming, you don't see any stars at all. A 10 minute exposure an
hour after sunset can look like a day shot, and if the moon is anything
more than crescent, you'll be limited to just a few minutes at best.
(By comparison, a full moon will look like a day shot in about 8-10
minutes at f2.8 at ISO 100.)

Picking a faraway place on a night with a new moon (or close to it on
either side) is best for getting the darkest skies, which make the
light from the stars is more pronounced. This may not be as easy as you
think. The photograph of the lit tents shown here was shot in Death Valley
(over 300 miles away from Las Vegas), which still had an illuminating
effect on the horizon. To illuminate the tent, I spent about 30 seconds
waving a flashlight around from inside the tent. This process is hard to
get right without overexposing the tent's fabric. Again, the benefits
of experimentation. This light also helped bring out detail on the ground.

While photos of nothing but star fields and trails are fascinating and
will impress your friends and neighbors, they can get pretty old pretty
fast if that's the only thing in your photo. These pictures are much
better with foreground subjects. Think daytime photography here: a
lake, an interesting tree, rock formation, or even your house. As you
experiment with various shots, the first thing that'll pop out at
you as you see the actual photos, is the direction of the star trails
themselves. This is never apparent when shooting the picture because
you don't actually see the stars move at all. Hence, the direction of
where the stars move will become an increasingly important element in
choosing your compositions.

On a perfectly dark nite under a totally new moon, you won't have much to
see other than stars. With no other light at alleven the ambient lightyou
can't see other objects at all in the foreground, which makes for limited
composition options. Here, it's common to get a silhouette of a tree
or a mountain. Although it's also fun to use a flashlight to illuminate
foreground subjects like a cactus. (I've even used the brake lights and
turn signals from my car to create colorful red and yellow bursting effects
on foreground objects.) Starting an exposure while a crescent moon is
just about to set can illuminate the foreground enough to have them lit
adequately, while permitting the exposure to continue (and get longer
trails) as it gets darker.

Most people use the Polaris, the northern hemisphere pole start as a
point of reference for composition decisions.
As the Earth rotates, stars will appear to spin around it, as shown in
the photo of Gusela Mountain, in the Italian Dolomites.
(Those who live in the southern part of the planet don't technically have
a southern pole star, but they do have "The Southern Cross." The southern
celestial pole can be located by extending the line from Crux Australis
(the Southern Cross) approximately 5 times the distance between the
two stars to reach the position of the pole. Alternatively, bisect the
'Pointers' and draw a line from that point to intersect the line from
the Southern Cross. They intersect at the pole.)

In choosing foreground subjects, you'll need to make sure you get them
in focus along with the distant stars, which often requires smaller
apertures. This works against you because the smaller aperture means less
light. Alas, your composition has trade-offs: composing a scene that can
uses a wide aperture, but still has interesting foreground subjects.
Generally, I compose scenes that have the closest subject to the lens
be at least 10 feet in front of me, if not further. I also use a wide
angle lens, so that I can get everything in focus at or f3.5 up to f5.6.
I try to avoid apertures greater than f8, which can yield a reasonable
picture if exposed for several hours or more. Again, I don't want to
raise my ISO setting, because the digital noise becomes intolerable.
However, doing so is a good way to experiment, and get a prototype of
what your composition will look like for a longer exposure at a lower ISO.

Because of the different sources of light and the great effects such
subtle changes can have on a very long exposure, you can't really
"calculate exposure times" here. The camera's light meter is irrelevant,
especially for exposures that are going to be well into the minutes, if
not hours. This requires setting the shooting mode to "manual" or "bulb"
and using a cable release. Some advanced cable releases have timers
built into them, whereas manual versions require you to push the
cable yourself. If you think that's not so bad, keep in mind that you're
going to have to be around (and awake) in several hours when you want
to release that button. Every camera manufacturer has different cable
releases to choose from, but after having done this for a while, I can
speak confidently that having one with a timer is well-worth the money.

At this juncture, you must now experiment and rely on trial and error
to learn the ropes. Just compose what you might think would work, release
the shutter, and go get coffee (or go to sleep).

It's tempting to want to use a higher ISO setting to brighten photos due
to the lower light conditions, but the side-effect of higher ISO is higher
noise. I use ISO 100 to keep the "digital noise" down, which is more
pronounced in darker areas of an image than lighter ones. Your mileage
may vary, as different camera manufacturers deal with noise differently.
Even though some cameras may perform better than others, all cameras will
produce much better images at lower ISOs than higher ones.

There are two ways to power cameras: batteries, and direct plug-in to a
wall socket. First, the batteries:

Almost any film-based SLR will expose long enough because battery
consumption is low. But digital cameras have a tougher time because
digital sensors eat power. Most consumer-brand cameras won't do much
more than about an hour or two on a single charge, but that gets better
as technology improves over time. You'll have to experiment. Higher-end
("prosumer") cameras can do much better, but on those cameras, there's
another factor: noise reduction. Long exposures often cause the camera
to take a second "dark frame" (internal) exposure for the same amount of
time as the first exposure, which it then uses as a mask to cancel out
the noise. You don't do this yourselfthe camera does it automatically
but it means that a 1-hour exposure requires 2-hours of battery life to
process the photo to the end. So, if you're going entirely from battery
power, you may be limited.

To get more images in less timeand to make the most of battery power
if you're not using a power adaptoropt for simpler night pictures
of shorter star trails, or just star fields. As stationary objects, it's
still a pretty amazing site.

For those using pro-level Canon equipment, I can vouch for my 1DsMark3
and 5DMark2, each of which will easily do a 2+ hour exposure, plus the
added 2nd "dark frame", on a full battery charge. I can't speak to other
camera models or brands.

If your camera comes with an direct AC power adaptor that allows you to
plug the camera in directly to the wall, you don't have a problem with
power. However, it means that your location is limited to one that has
proximity to an outlet. To optimize this flexibility, get one of those
100-foot long extension cables at a home supply store or even drug stores.
Not all cameras have such an adaptorespecially the lower-end consumer
brandsso if you're in the market for a new camera, be sure to look for
this.

When staying in hotels, I often choose rooms that face towards the darkest
part of the sky and that have the least amount of ambient light (usually
the decorative hotel lights). This way, I plug into the wall, and place
the camera (on tripod) either on the balcony, or shoot through the window.
When camping, I use an AC power adaptor plugged into the car or a nearby
power supply. (A house, cabin, etc.) Obviously, this may not always be
possible. In this case, true night-photo nuts go out and buy generators
or battery packs that can keep a good exposure going for quite some time.

A common problem with night photography is dew fogging the glass because
ambient air is warmer than the lens itself. What causes dew isn't the
temperature, it's the amount of humidity in the air (though cold fronts
tend to have drier air). Metal lenses will always be colder than the
ambient temperature, but even lenses with plastic barrels can suffer
from the problem simply because of the glass components inside the lens.
The solution is keep the lens warmer than the air. How you do that is
the challenge. The most fool-proof way is to get a bulky battery
pack and wire it up to your lens. Places that sell astronomy equipment
make these for larger telescopes, but they are too large for normal
camera equipmentit'd be like shooting a mouse with an elephant gun.
The solution "works" and won't damage anything, but it's an overkill.